Impact of Hydrogen in-fiber and out-fiber Diffusion on central wavelength of UV-written Long Period Grating

Hydrogen loaded standard fibers are routinely used for writing fiber gratings as the diffusion of molecular hydrogen strongly enhances the fiber photosensitivity [1]. Out-fiber diffusion of molecular hydrogen occurs when fiber is removed from hydrogen pressure. If the out-fiber diffusion after few hours at room temperature has negligible effect on the value of the initial Bragg wavelength for classical Bragg gratings (FBG), for long-period gratings (LPG) we show that this out-diffusion considerably changes the value of the initial central wavelength. Furthermore, in a reciprocal way, the in-fiber diffusion of hydrogen in LPG can induce strong shift of the central wavelength. In this paper, we present for the first time to our knowledge, both theoretical and experimental studies on the impact of hydrogen in-fiber or out-fiber diffusion on Long-Period Gratings.